This invention was made during a search for a way to improve solubility of granular laundry detergent products and prevent clumps of detergent from remaining in the washer and on washed clothes. Such clumps, which may appear as solid white masses ranging from about 5 to 40 millimeters in diameter and about 2 to 10 millimeters in length, have occurred during cold water washes when the order of addition to the washing machine is laundry detergent product first, clothes second, and water last. It has been found that the primary contributors to this solubility problem are the sodium carbonate and sodium phosphate in the granular laundry detergent. It has been discovered that surprisingly low levels of potassium salts can be included in the granular detergent composition to improve solubility and eliminate or reduce this clumping problem.
Sodium carbonate and sodium pyrophosphate have been used in granular detergent compositions (for example, U.S. Pat. No. 4,299,717, Cottrell et al, issued Nov. 10, 1981). Potassium salt has been substituted for sodium salt to eliminate giant micelles of sodium salt of washed fatty acid in the washing liquid (Japanese Patent Application 61164000). The solubility of a solid is lower in a solution of a salt containing a common ion than in pure water (Chemical Principles, 4th ed., Masterton Slowinski, W. B. Saunders Co. 1977, pg. 435). Lastly, a water-softening composition comprising tetrasodium pyrophosphate and an alkaline material selected from the group consisting of trisodium phosphate, sodium hydroxide, sodium carbonates, potassium hydroxide, potassium carbonates, soap and sodium silicate is disclosed in U.S. Pat. No. 2,381,960, Johnson, patented Aug. 14, 1945. However, it has not been disclosed that low levels of potassium salt can be added to a granular laundry detergent composition or additive containing sodium carbonate and sodium phosphate to improve solubility of the composition in water.